Conventionally, in the process industry field, pressure transmitting devices have been used in order to control processes by detecting, for example, the amounts of variations in processes. Pressure transmitting devices are also known as pressure sending devices. These pressure transmitting devices are able to measure amounts of process variations, such as in pressure, flow rates, fluid levels, specific gravities, and the like, through measuring differential or absolute pressures between two points. Typically, when measuring the amounts of process variations using pressure transmitting devices, that which is to be measured is introduced into the pressure transmitting device through a thin tube, known as a pressure guiding tube, from, for example, both sides of a differential pressure generating mechanism, such as an orifice, that is disposed in a process pipe wherein that which is to be measured, such as a liquid, is flowing.
In this type of device structure, blockages in the pressure guiding tubes may result from the adherence, to the interior of the pressure guiding tubes, of solid objects, or the like, according to that which is being measured. If a pressure guiding tube becomes completely blocked, then it becomes impossible to measure the process variations accurately, which can have a serious impact on the plant. However, because pressure is still transmitted to the pressure transmitting device up until the point wherein the pressure guiding tube becomes completely blocked, the impact of the blockage tends to not appear in the process variation measurement values. Remote seal-type pressure transmitting devices wherein pressure guiding tubes are not required have been developed in response to this type of problem. However, an extremely large number of plants measure process variations using pressure guiding tubes, and thus there is the need to be able to perform pressure guiding tube blockage diagnostic functions on-line.
Conventionally, the technologies disclosed in Japanese Examined Patent Application Publication H7-11473 and Japanese Patent 3139597 have been known as technologies for diagnosing the state of pressure guiding tube blockage. The fault detecting device disclosed in Japanese Examined Patent Application Publication H7-11473, as illustrated in FIG. 20, inputs, over a specific time interval, a signal regarding that which is being measured, detects the maximum amplitude W of variation in the inputted signal within a given time interval, and compares the detected maximum variation amplitude W to a threshold value that has been set in advance, to determine that there is a fault in the signal if the maximum variation amplitude W is smaller then the threshold value. In FIG. 20, Smax is the maximum value for the signal, and Smin is the minimum value for the signal. Japanese Examined Patent Application Publication H7-11473 discloses that pressure guiding tube blockage status diagnosis can be performed through the application of this fault detecting device.
In the fault detecting device disclosed in Japanese Examined Patent Application Publication H7-11473, if the set time period that is interval for detecting the maximum variation of the signal is sufficiently much longer than the period of the variation of the signal, then the effect will be to detect a maximum variation amplitude W from among the difference between adjacent maximum values and minimum values. Additionally, if the aforementioned set time period is shorter than the period of the variation of the signal, then the effect will be to detect the maximum variation amplitude W simply within the set time period. In particular, if the signal is sampled discontinuously, the aforementioned set time period is set so as to detect amount of variation in a single sampling interval, then the effect will be to detect a difference value (that is, the differential value) of the signal.
The blockage diagnosing device disclosed in Japanese Patent 3139597 detects the fluctuation (variation) in pressure of that which is being measured, and evaluates that a blockage has occurred in the pressure guiding tube when the difference between the detected magnitude of fluctuation and the magnitude of normal fluctuation exceeds a value that has been set in advance. In Japanese Patent 3139597, a pressure differential signal and a difference signal between an upper peak (the maximum value) and a lower peak (a minimum value) for pressure are given as examples of signals indicating fluctuations in pressure. The signal for the differential in the pressure, disclosed in Japanese Patent 3139597, corresponds to the difference value of the signal disclosed in Japanese Examined Patent Application Publication H7-11473, and the difference signal disclosed in Japanese Patent 3139597 corresponds to the maximum variation amplitude W disclosed in Japanese Examined Patent Application Publication H7-11473. Consequently, the technology disclosed in Japanese Examined Patent Application Publication H7-11473 and the technology disclosed in Japanese Patent 3139597 can be said to be based on the same technical concept.
As described above, in the technology disclosed in Japanese Examined Patent Application Publication H7-11473 and Japanese Patent 3139597, the state of blockage of a pressure guiding tube is diagnosed based on the magnitude of the fluctuation in pressure, a threshold value to serve as a reference in the diagnosis is required and at the time of the diagnosis. In the technology disclosed in Japanese Examined Patent Application Publication H7-11473 and Japanese Patent 3139597, there is a problem in that this threshold value must be adjusted appropriately in accordance to the magnitude of the pressure, and a problem in that time and specialized knowledge is required to adjust the threshold value.
For ease in understanding, the conventional problem areas will be explained assuming extreme numerical values. For example, even if a fluctuation of ±3 kPa is normal in a pressure value of 100 kPa, a fluctuation of ±3 kPa could not be considered normal in a pressure value of 5 kPa. Consequently, it would be inappropriate to use the same threshold value when the pressure value is 100 kPa as when the pressure value is 5 kPa, and the threshold value must be made smaller for the case of the pressure value of 5 kPa.
Additionally, one cannot diagnose the same state of blockage in a case of a 2 kPa fluctuation instantaneously from a pressure of 80 kPa to 82 kPa, in a state wherein, for example, the pressure would be about 100 kPa if smoothing were performed, as in a case of a 2 kPa fluctuation instantaneously from a pressure of 80 kPa to 82 kPa, in a state wherein the pressure would be about 60 kPa if smoothing were performed. Consequently, the same threshold values would not be considered to be appropriate in both of these cases.
As is clear from the explanation above, in the technologies disclosed in Japanese Examined Patent Application Publication H7-11473 and Japanese Patent 3139597, it is necessary to adjust the threshold value that is the reference for the diagnosis.
The present invention is to solve the problem areas set forth above, and the object thereof is to provide a pressure guiding tube blockage diagnosing device and blockage diagnosing method able to reduce the need to change the threshold value that is the reference for the diagnosis.